Various mechanisms are known for moving sheet material, such as paper, or the like, which material is imprinted at a printing station or is otherwise acted upon. Previous mechanisms have included pin type feed devices, which include pins carried on an endless chain or belt, with the belt being driven by an appropriate drive sprocket. The pins move into apertures in the paper for positive advancement thereof. An example of such a mechanism appears in U.S. Pat. No. 3,507,431. Through experience with the mechanism taught in the aforesaid patent and with other prior art mechanisms, a number of problems have arisen.
The pins secured to an endless belt are subject to shifting longitudinally in the plane of the belt, and/or to shifting out of the plane of the belt as the pins move the sheet material. Such misalignment of the pins tend to cause the sheet material to tear or separate from the pins or to cause improper orientation of the sheet material resulting in misaligned imprinting. Furthermore, prior art mechanisms often do not provide positive guidance and support for the belt carrying the pins during the sheet material engaging pathway; which contributes to the aforesaid undesired motion of the pins. Further still, the prior art has not provided adequate means for adjusting a plurality of tractor mechanisms with respect to each other for various width sheet materials. Further still, the prior art has required a driven sprocket and an idler sprocket for cooperating with the endless belt which carries the pins. Adjustment of the spacing between the sprockets or otherwise ensuring proper tensioning of the pin carrying belt has proven quite difficult where two rotated sprockets are involved. Furthermore, with two round, spaced apart sprockets, there is not adequate guidance and support for the pin carrying belt over its full pathway. Additionally, it has been found that there has been undesired play between the means which drives the drive sprocket for the pin carrying belt and the sprocket itself. This has caused improper positioning of the sheet material at a printing station, or the like, which has introduced undesired errors in the location of the imprinting on the sheet material.
Although previous tractor mechanisms are capable to move sheet material as desired, the above noted drawbacks have resulted in the development of the present invention.